Recently, deterioration of many structures constructed in high growth period, and a research on a method of evaluating the health of the structure is being actively carried out with the government taking the lead. Conventionally, an electrical method using a wire resistance distortion gauge has been the primary method of measuring the distortion of the structure, but had problems of reliability, lightening strike, electromagnetic noise and the like, and thus attention is given to measurement (sensing) by optical fiber that has none of the above problems. It is also noted that the transmission loss of the optical fiber is small so that the remote measurement is possible.
In the optical fiber sensing field, the fiber-Bragg grating (FBG) is used in distortion measurement of the structure in combination with a wide band light source or a wavelength variable light source.
As shown in FIG. 16, the FBG has the refraction index of the core 103c of the optical fiber changed at a regular interval D, and for the light entering the optical fiber, reflects the light of wavelength defined by the interval D and the refraction index of the core 103c and transmits the other wavelengths. Thus, the reflected wavelength is displaced by the change in interval D due to distortion over the FBG 103 or the change in refraction index due to temperature. Normally, the temperature characteristic of the FBG 103 is about 0.01 nm/° C., and the distortion characteristic is about 0.0012 nm/με. Such characteristics are used in temperature measurement and distortion measurement.
That is, when FBG is applied with stress and expanded or contracted (when FBG is distorted), the grating distance moves and the reflected wavelength of the FBG changes. By attaching the FBG to the measurement sites of the structure, and entering the light of wide band light source or the wavelength variable light source enter therein and measuring the reflected wavelength, the distortion of the FBG, that is, the distortion of the structure can be measured. Further, by measuring at high-speed, the vibration of the structure can be measured and thus is put to practical use as a seismograph.
For instance, as a method of measuring the reflected wavelength of the FBG, a method of using the wide band light source for the light source and using the Fabry-Perot filter (hereinafter referred to as FP) is proposed in patent document 1.
FIG. 9 is a view showing a schematic configuration shown in patent document 1.
The light from a wide band light source 101 is passed through a light branching unit 102 and an optical fiber 108 to an FBG 103. The light of specific wavelength is reflected at the FBG 103, and such reflected light is passed through the optical fiber 108, the light branching unit 102, and an FP filter 104 to a light receiving unit 105. The light that has reached the light receiving unit 105 is converted to a current therein.
An FP filter 104 is an interferometer that uses etalon plates, and drives the distance between the two etalon plates facing parallel to each other by a piezoelectric element to operate the interfering distance. The light passing through the FP filter is a light of specific wavelength defined by the interfering distance. The reflected light of all the wavelength band output from the wide band light source 101 is detected by changing the interfering distance to an arbitrary length. The piezoelectric element is able to set the wavelength with respect to the driving voltage since the driving distance is defined by voltage. By detecting the intensity of light of the light receiving unit 105 corresponding to the driving voltage, the reflectance spectrum from the FBG 103 can be detected and the reflected peak wavelength can be specified.
Moreover, in patent document 2, an FBG sensing device that uses the FP filter as the light source to be the wavelength variable light source is proposed. This device detects the reflectance spectrum from the FBG from the amount of received light corresponding to the driving voltage of the FP filter and specifies the reflected peak wavelength.
The device using such FP filter requires a driving unit for driving the FP filter and thus causes problems in terms of reliability. Further, since the intensity of light of all the wavelengths corresponding to the spectrum of the measuring region must be detected, the number of data becomes a vast amount, and a high-precision arithmetic processing apparatus becomes necessary. The use of such arithmetic processing apparatus becomes the cause of degradation of reliability. Further, the FP filter has wavelength interpolation frequency of about 10 Hz to 100 Hz. According to non-patent document 1, the speed of response of the wavelength measurement is obtained to be 100 to 200 Hz when assuming the vibration measurement during earthquake. Therefore, the device using the FP filter is not suitable for vibration measurement.
In patent document 3, the use of two narrow band filters without the driving unit in the wavelength measurement unit is proposed.
FIG. 10A shows the wavelength measurement unit in the FBG sensing device shown in patent document 3.
It is configured by a wide band light source 201, a 2×2 coupler 202 for branching the light, an FBG 203 that acts as a distortion sensor, a 2×2 coupler 207 for branching the light, a narrow band filter 204 having wavelength dependence, a light receiving unit 205 and an optical fiber 208.
The light output from the wide band light source 201 is transmitted through the optical fiber 208 via the 2×2 coupler 202 and guided to the FBG 203. In FBG 203, the specific wavelength is reflected, which then is passed through the optical fiber 208, returned to the 2×2 coupler 202, branched and guided by the 2×2 coupler 207, and further branched into two parts of A and B, and passed through the narrow band filter 204 and converted to an electrical signal in the light receiving unit 205.
The narrow band filter 204 has wavelength dependence as shown in FIG. 10B, and can detect the reflected wavelength of the FBG 203.
Generally, the SLD (Super Luminescent Diode) light source or ASE (Amplified Spontaneous Emission) light source is used for the wide band light source in these devices.
The ASE light source generates a spontaneous emission light of wide band and high output by entering the excited light of a specific wavelength to the optical fiber added with erbium. An output of about 100 times (improvement of 20 dB) the SLD light source is obtained. The use of light source of higher output allows the measurement sites (FBG) with respect to the light source to be arranged at a more remote location.
As noted above, the transmission loss of the optical fiber is small, but the amount of loss is about 0.25 dB/km. If the output of the light source is improved by 100 times, that is 20 dB, the distance in which the remote measurement is possible can be extended by about 40 km (80 km forward and backward: 80 km×0.25 dB/km=20 dB).
However, the output of the ASE light source which is about −10 dB/nm or so is not a sufficiently large value. In the configuration shown in FIG. 10A, since the narrow band filter 204 has the wavelength dependence as shown in FIG. 10B, the attenuated light is entered to the light receiving unit 205. Generally, the amount of attenuation at the narrow band filter 204 is about 20 dB at maximum (sleeve of the spectrum of FIG. 10B). The minimum light receiving sensitivity of the light receiving unit 205 is about −50 dB, and thus taking the output of the light source and the amount of attenuation of the narrow band filter 204 into consideration, the allowable range of the transmission loss by the optical fiber is only about 20 dB. The allowable range is further reduced when the loss of the other components and the margin of the system are taken into consideration. That is, measurement of a sufficiently remote distance cannot be performed.
When performing the distortion measurement, the temperature characteristic must also be considered in addition to the distortion characteristic, and the amount of displacement of the reflected wavelength caused by distortion change must be subtracted from the amount of displacement of the reflected wavelength caused by temperature change. Thus, when performing the distortion measurement, the FBG having two different reflection wavelength bands must be used.
Non-patent document 2 describes an FBG sensing device, using the wide band light source, for carrying out the FBG having a plurality of different reflection wavelength bands with one optical fiber.
FIG. 17 is a view showing a configuration of the FBG sensing device described in non-patent document 2.
The light from a wide band light source 101 is transmitted through a light branching unit 102, transmitted through an optical fiber 108, and reached to an FBG 103. The light of a plurality of different wavelengths is reflected in the FBG 103, which is then transmitted through the optical fiber 108, transmitted through the light branching unit 102, and reached to a wavelength detector 110, where the reflected wavelength is detected.
Thus, by using the wide band light source 101, the FBG having a plurality of different reflected wavelengths can be arranged in one optical fiber, and a plurality of distortion and temperature amounts can be simultaneously measured.
Generally, the SLD (Super Luminescent Diode) light source or the ASE (Amplified Spontaneous Emission) light source is used for the wide band light source, but the wavelength band thereof is about 30 nm to 50 nm.
The number of FBGs with respect to the band of the wide band light source 101 will now be explained.
Since the temperature characteristic is about 0.01 nm/° C. and the distortion characteristic is about 0.0012 nm/με, as noted above, the usage band of one FBG 103 requires 0.01 nm×50° C.+0.0012 nm×2000 με=2.9 nm when measuring the amount of distortion of ±1000με in the temperature region of for example, 0° C. to 50° C. Further, the reflection wavelength band of the FBG 103 is about 0.2 nm at full width at half maximum, and in consideration thereof, the usage band of one FBG 103 requires about 3.1 nm. Generally, in consideration of the margin, the band of greater than or equal to 4 nm is used for one FBG. Therefore, the number of FBGs 103 of different reflection wavelength band is about ten.
When performing the distortion measurement, two FBGs 103 of for distortion measurement and for temperature compensation are required, as noted above, and the number of distortion measurement becomes five points. The number of points desired for distortion measurement may be less than or equal to ten points, but may be a several dozen points, and thus is not a sufficient number of points.
As a means for increasing the measurement sites, increasing the band of the wide band light source is easily contrived, and a light source having a band of about 100 nm is already commercially available, but is not actively used since the types of FBG 103 increases. Presently, it is generally handled by increasing the number of optical fibers.
Reference is made to patent document 1 (Japanese Laid-Open Patent No. 2003-21576), patent document 2 (Japanese Laid-Open Patent No. 2001-511895), patent document 3 (Japanese Laid-Open Patent No. 2000-223761), non-patent document 1 (Akira Mita, 25th Light Wave Sensing Engineering lecture papers, June, 2000 LST 25-16, PP. 111-116), and non-patent patent document 2 (published December, 1995 in Application to Optical Measurement/Sensor, recent references of Optronics Optical Communication Technique by Shinji Yamashita et al.)